Yes, the nice neighbor thing to do would be to sweep up leaves instead of blowing them. If washing down the deck, try to do it at a time when it's unlikely your neighbors below will be out. It might be nice to warn them in advance.

Generally you shouldn't be doing something that will cause things to fall on to their deck. Obviously you can't control dust or rain water from falling between the boards.

It would be good to speak to the person below about your concerns. Outdoor carpet on your deck might help. just shake or vacuum.

If you're blowing debris, maybe offer to blow theirs afterwards to remove what ended up down there.

Very thoughtful of you to think of them.

A broom to sweep the larger debris, an outdoor vacuum will pick up the dust and grime. Then a damp mop will get the decking cleaner. Since it’s outside there’s not a need for it to be as clean as the inside.